An example reductant is an aqueous urea solution optimally blended with 32.5% urea and 67.5% deionized water, and is used in exhaust systems equipped with an SCR apparatus to lower NOx exhaust emissions. In an SCR apparatus, the urea is converted into ammonia, which is stored in the ceramic walls of the catalyst of the SCR apparatus. The purpose of the urea injection into the SCR apparatus is to control the ammonia storage to assist with NOx conversion. Due to the exposure in the exhaust gas pipe of the engine by high temperature, lubricating oil burned during combustion, sulfur from the certain fuels, and unburned hydrocarbons that are absorbed by the catalyst of the SCR apparatus, there is an irreversible natural aging of the catalyst. If the catalyst of the SCR apparatus ages to a degree of having reduced ammonia storage capacity, the tailpipe NOx emissions will increase and ammonia slip will occur. The more the SCR apparatus is degraded, the greater the amount of ammonia slip. Accordingly, the art of controlling NOx emissions, and ammonia slip, in the exhaust of internal combustion engines will be improved by providing a control system that can adjust the dosing of the reductant as the catalyst of the SCR apparatus ages.